I'd connect a relay in between the back-lights and the topper
that's rated for the same voltage as the back-lights.
Leave the power connections for the topper as normally routed but
enable he electronics in the relay to switch it on and off.
That way, the topper will go nuts during a win in game play but not draw any juice
from the back-lights as they're very sensitive to power draws - not to mention extreme line noise
which can severely hamper and thoroughly and thoughtlessly reduce the speaker's "enhanced"
sound outputs to a squeal-like mush...
I'd take it one step further down the road and connect the belly door and award
glass fluorescent lighting in the same manner so the machine
pretty much looks like it's going to blow up....
I did it to my pinball machine but used a police roof light...
it certainly makes the blood flow better when everything is like a light explosion and chaotic as heck!
Don't forget the confetti cannon that explodes when connected to the Jackpot Bell!